1. Field of the Invention
This invention relates to a thermal head that is modified on the surface thereof to be kept low in surface tension without inhibiting thermal conduction, and particularly relates to a thermal head that maintains excellent perforation property for a long period of time when used for perforating heat sensitive stencil sheets.
1. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
As one of conventional methods of perforating heat sensitive stencil sheets, known is a stencil making method using a thermal head which is, in general, also called thermal printing head. In this method, the thermoplastic resin film face of a heat sensitive stencil sheet is brought into contact with a thermal head, for melting and perforating the thermoplastic resin film in an area corresponding to an image area of an original, by mean of heat of the thermal head.
However, if this method is used to continuously make stencils, there is a problem that the melt of the film is deposited on the surface of the thermal head to gradually degrade thermal perforation property of the thermal head.
In general, thermal heads can be structurally classified into thin film type, thick film type, semiconductor type, etc. The thin film type thermal head generally has, as shown in FIG. 1, a layered structure consisting of an insulating substrate 1, heat-generating resistor 2 formed on the insulating substrate 1, an electroconductive layer 3 connected with the heat-generating resistor 2 for supplying electric power to it, and a protective layer 4 covering the heat-generating resistor 2 and the electroconductive layer 3. The thick film type thermal head generally has, as shown in FIG. 2, a similar layered structure consisting of an insulating substrate 1, a heat-generating resistor 2 and an electroconductive layer 3 formed on the insulating substrate 1, and a protective layer 4 covering the electroconductive layer 3 and the hear-generating resistor 2. Therefore, the surface of a thermal head generally means the surface of the protective layer 4.
As the material of the protective layer 4, an inorganic material having relatively good thermal conductivity such as Ta2O5, SiO2, SiON or Si3N3 is used. However, since these inorganic materials have high surface free energy, they have high surface tension, and thus have such a nature that the melt of the film is likely to be deposited on the surface of the thermal head.
To solve the above problem, it is proposed to coat the film surface of a heat sensitive stencil sheet with a releasing agent (JP-A-61-170392) or to let a heat sensitive stencil sheet contain a releasing agent in the porous substrate or adhesive layer thereof (JP-A-2-255384). However, since these methods have a releasing agent applied to a heat sensitive stencil sheet, the have such disadvantages that the stencil sheet production process is complicated to raise production cost and that uniform performance is difficult to obtain.
To overcome these disadvantages, it is proposed to further form a water-repellent, oil-repellent and heat-resistant resin layer on the surface of the thermal head, i.e., the protective layer 4, for preventing the deposition of the melt of the film onto the surface (see JP-Y-4-7967, JP-A-60-2382, JP-A-60-178068, JP-A-62-48569, etc.). The resin layer is typically made of a fluorine resin such as Teflon (trade name of Du Pont: polytetrafluoroethylene). For coating the surface of a thermal ahead with such a fluorine resin, it is usually necessary to prepare a dispersion containing 50 to 60% solid polytetrafluoroethylene, to coat the surface of a thermal head with the dispersion, to preliminary dry and to heat up to about 350xc2x0 C.
The fluorine resin layer is excellent in making the surface of a thermal head lower in surface tension, but the treatment process (heating process) thermally loads the electronic parts associated with the thermal head. So, the method cannot be said to be a simple and proper treatment method. Furthermore, the fluorine resin has such a problem that bonding strength to vitreous materials such as the protective layer is not sufficient.
Moreover, since the above resin layer is a coating layer of resin, even if thin coating is made, the thickness becomes about 1 xcexcm, to inhibit the efficient thermal conduction from the heat-generating resistor to the surface. There is also a limit in making the thickness of the resin layer uniform for enhancing the surface smoothness, and the actually obtained thickness and surface roughness are on the order of microns.
Above all, in the case where such a thermal head is used to process heat sensitive stencil sheets into stencils, there is a problem that the roughness of the resin layer formed on the surface of the thermal head inhibits close contact between the thermal head and the heat sensitive stencil sheet, thereby lowering the thermal conductivity. As a result, uniform perforation of the heat sensitive stencil sheet cannot be ensured.
Furthermore, as other methods for making the surface of a thermal head lower in surface tension, proposed are a technique comprising the step of coating the surface of the protective layer with a fluoroalkyl group-containing silane compound for forming a water-repellent, oil-repellent film, and a technique comprising the steps of pre-treating the protective layer using, for example, silicon oxide for forming an undercoating layer and forming said water-repellent, oil-repellent film on the undercoating layer, to make a two-layer structure, in order to improve the bonding strength between the water-repellent, oil-repellent film and the protective layer (Japanese Patent Application No. 2000-30694). The former method is a very simple and advantageous method for making the protective layer lower in surface tension without inhibiting the thermal conductivity since the obtained water-repellent, oil-repellent film is a uniform film of molecular level by virtue of properties of the fluoroalkyl group-containing silane compound. However, the method may be insufficient in performance in applications that require film durability such as scratch resistance. On the other hand, the latter method has a disadvantage that production cost is raised since the work basically consisting of two steps complicates the thermal head production process, though it can be expected that durability will be higher compared with the former method.
The object of this invention is to overcome the problems of the above-mentioned prior art, that is, to lower the surface tension of the protective layer by a simple method for preventing the deposition of the melt on the thermal head for a lone time while maintaining the thermal conductivity from the heat-generating resistor to the surface of the thermal head and the smoothness of the protective layer.
According to this invention, the above object can be achieved by a thermal head which comprises an insulating substrate, a heat-generating resistor formed on the insulating substrate, an electroconductive layer connected with the heat-generating resistor for supplying electric power to it, and a protective layer formed on the heat-generating resistor and the electroconductive layer, wherein said protective layer is treated on the surface thereof with a dry film of a surface-treating agent containing a chlorosilyl group-containing compound and a fluoroalkyl group-containing silane compound.
The surface-treating agent can be produced, for example, by a method of dissolving a chlorosilyl group-containing compound and a fluoroalkyl group-containing silane compound into an organic solvent. Then, the surface-treating agent can be coated on the surface of the protective layer of the thermal head and dried, to form a water-repellent, oil-repellent film on the surface.
Thus, according to another aspect of this invention, there is provided a surface-treating agent containing a chlorosilyl group-containing compound and a fluoroalkyl group-containing silane compound in an organic solvent, for making tie vitreous surface of a thermal head water-repellent and oil-repellent.
According to a further other aspect of this invention, there is provided a method of treating a surface of a thermal head having an insulating substrate, a heat-generating resistor formed on the insulating substrate, an electroconductive layer connected with the heat-generating resistor for supplying electric power to it, and a protective layer formed on the heat-generating resistor and the electroconductive layer, which comprises the steps of coating the surface of the protective layer with said surface-treating agent and drying, in order to modify the thermal head to be water-repellent and oil-repellent on the surface of the protective layer.
The protective layer of a thermal head is usually made of a vitreous material containing Ta2O5, SiO2, SiON or Si3N3, etc. So, if a fluoroalkyl group-containing silane compound that is a water-repellent, oil-repellent and heat-resistant compound is used as a surface-treating agent, the surface of the protective layer can be modified into a water-repellent, oil-repellent and heat-resistant surface. The fluoroalkyl group-containing silane compound is hydrolyzed with water in a solution, moisture in air or moisture adsorbed on a surface of inorganic materials, to produce highly reactive silanol groups (Sixe2x80x94OH). The silanol groups are reactive groups that can be adsorbed by or chemically bonded to the surface of inorganic materials. So, if they are used for treating the surface of the protective layer of the thermal head, which is composed of a vitreous material, the surface of the protective layer can be chemically modified. The surface-treating agent of this invention has a chlorosilyl group-containing compound coexisting with the fluoroalkyl group-containing silane compound. The chlorosilyl group-containing compound is hydrolyzed with water in a solution, moisture in air or moisture adsorbed on a surface of inorganic materials to produce highly reactive silanol groups (Sixe2x80x94OH), like the fluoroalkyl group-containing silane compound, and byproduces hydrochloric acid to promote the hydrolysis of the fluoroalkyl group-containing silane compound. At the same time, it is combined with the hydrophilic groups (xe2x80x94OH groups, etc.) on the surface of the protective layer or reacts with the silanol groups (Sixe2x80x94OH) of the fluoroalkyl group-containing silane compound, to form a polysiloxane. Therefore, production of the water-repellent, oil-repellent film is promoted, and the film is strengthened.
As described above, according to this invention, a very durable water-repellent, oil-repellent film that is mainly composed of silicon oxide and also contains fluoroalkyl groups can be simply formed on the surface of the protective layer of the thermal head by one step based on a sol-gel method, and excellent properties can be maintained for a long period of time. Furthermore, it is confirmed that the surface treatment of this invention can improve the contact angle of the surface of the protective layer against water up to 95xc2x0 or more. Moreover, since the silanol groups are combined with the hydrophilic groups such as xe2x80x94OH groups existing on a solid surface, every vitreous surface can be modified to be water-repellent and oil-repellent as far as it is composed of a material capable of providing said hydrophilic groups.